At high densities, various physical processes become important which can affect the ionization and thermal balance. These include:
Lowering of the continuum, in which collisional ionization from highly excited levels (i.e. superlevels) results in a net reduction in the effective reccombination rates. This effect is only included for H and He-like ions in our calculations. This process is most important at low temperatures. A competing effect is collisional deexcitation from superlevels, but this turns out to be less important than continuum lowering.
3-body recombination results in a net increase in total recombination rate. In our models we include collisional ionization and 3-body recombination from essentially all levels, although this process is generally more important for levels closest to the continuum.
At high densities the spectroscopic level populations in the recombining ion can approach their LTE values, leading to enhanced collisional ionization from these levels and a decrease in the total recombination. This turns out to be unimportant for most ions at the densities and temperatures we consider.
In a photoionized plasma the incident photon flux must be very large if the density is high and the ionization parameter is within the conventional range. Such high photon fluxes can lead to large enhancements in the recombination rate via stimulated recombination.